The present invention relates to power conversion apparatus and methods, and more particularly, to apparatus and methods for producing polyphase alternating current (AC) power from a direct current (DC) source in apparatus such as uninterruptible power supplies (UPS). UPSs are power conversion devices that are commonly used to provide conditioned, reliable power for computer networks, telecommunications networks, medical equipment and the like. UPSs are widely used with computers and telecommunications devices, including but not limited to personal computers, workstations, mini computers, network servers, routers, switches, disk arrays and mainframe computers, to insure that valuable data is not lost and that the device can continue to operate notwithstanding temporary loss of an AC utility source. UPSs typically provide power to such electronic equipment from a secondary source, such as a battery, in the event that a primary alternating current (AC) utility source drops out (blackout) or fails to provide a proper voltage (brownout).
Referring to FIG. 1, a conventional pulse width modulated (PWM) polyphase (e.g., three-phase) UPS generates a polyphase AC output at a load 20 from a DC voltage V.sub.DC generated between positive and negative voltage rails by a DC voltage generating circuit 10. Depending on the configuration of the UPS, the DC voltage V.sub.DC may be generated in a number of different ways. For example, the DC voltage V.sub.DC may be directly generated by battery or, as shown, by DC/DC conversion of a lower battery voltage V.sub.B generated by a battery 12. In some configurations, the DC voltage V.sub.DC may be generated from an AC source, or from a combination of an AC source and a DC source. The DC voltage V.sub.DC is inverted by a polyphase PWM inverter circuit 30, which typically includes a plurality of switches (e.g., transistors) that are selectively operated to synthesize AC voltage waveforms from the DC voltage V.sub.DC. Additional filtering may be provided to reduce harmonics generated by the switching operations.
In order to improve control of such PWM inverter circuits, so-called "space vector modulation" (SVM) techniques have recently been introduced. SVM techniques are commonly used in polyphase induction motor control applications, and typically involve transforming a "reference" voltage vector to a vector in a "d-q" or "space vector" coordinate domain, which is in turn used to generate a PWM switching pattern. In a closed loop motor control application, for example, the reference voltage may be a control voltage derived from an error voltage generated by a speed or voltage control loop. The vector produced by the space vector (d-q) transformation is mapped to various logical values of switching signals applied to the switching devices of a PWM inverter circuit based on its orientation in the d-q plane. A variety of mappings may be utilized, as described, for example, in the article by Hava et al. entitled "Carrier-Based PWM-VSI Overmodulation Strategies: Analysis, Comparison and Design,"IEEE Trans. on Power Electronics, Vol. 13, No. 4, pp. 674-689 (July 1998).
UPS's are often used to serve unbalanced polyphase loads, particularly in large-scale operations in which a single high-capacity UPS may be used to provide power to a variety of single-phase loads connected to individual phases. Although loads may be configured with a view toward minimizing phase imbalance, practical constraints in load location and use often result in significant load imbalances. Accordingly, it is generally desirable to compensate for imbalances in the loads presented to the phases of a UPS. Unfortunately, however, conventional polyphase UPS control schemes often provide poor regulation of individual phases.